Dual Projector systems that are used on very large curved screens for both 3D and non-3D digital cinema applications cannot be aligned in, or overlaid on, a pixel-for-pixel manner due to optical geometric errors. Although in theory, digital image warping may be used to correct for such misalignment, in practice electronic warping at 4K and high frame rate speeds is very difficult and expensive. This is because in order to avoid image quality degradation, warping requires many more pixels on the imager panel than are displayed by the image content (e.g. in digital cinema, the image content can be 4096×2160 pixels and is encrypted).
Crossed (i.e. X) mirrors as well as single mirror systems have been used to co-locate the optical projection axes of dual projection lenses more closely in order to minimize alignment errors. For example, it is known to use crossed mirror systems in 3D CAD using rear projection flat screens having short throw distances to preserve alignment and minimize optical geometric distortion between left and right eye images. Unlike the flat screens used for 3D CAD applications, the large high gain screens used for 3D cinema are almost always tightly curved. At 4K image resolution, projection on conventional short throw, very large tightly curved screens, results in unacceptably low image registration, particularly for 2D projection, and consequent lack of sharp resolution (as measured by the modulation transfer function (MTF)).